The present invention relates to a fluid valve, and more particularly, to a fuel injector for an internal combustion engine which meters fuel with injector stroke.
Fuel injectors are used to precisely and accurately regulate the flow of fuel into the combustion chamber of an engine based on several parameters, such as engine speed, engine load and fuel density. Typically, an electronic fuel injection system uses a solenoid-actuated needle valve comprised of a needle and a seat to allow passage of fuel into the combustion chamber. The needle valve cooperates with an orifice to regulate the flow of fuel through the injector.
In known injectors, the needle is lifted away from the seat by an electronically controlled solenoid (not shown) to open the orifice. The amount the needle is lifted away from the surface is called injector lift or stroke. The maximum static flow rate of a fluid through a passage is determined by the sum total of all of the pressure drops experienced by the fluid as it travels through the injector. There are typically two areas in the conventional fuel injector where the fluid flow can meet significant pressure drops. The pressure drops at these two areas could ultimately control the maximum static flow rate.
One area in a conventional injector is the cross-sectional area at the orifice. The orifice, conventionally, may be provided as a precise hole in the seat, or by a thin edge orifice plate placed downstream of the seat. The other area is represented by an annular frusto-conical area extending between the seat surface and the end surface of the needle. This annular frusto-conical area is directly related to the injector stroke.
The relationship between injector stroke and flow rate for a given orifice size where the orifice is used to regulate the maximum flow rate is termed lift sensitivity. The flow rate changes significantly at small injector strokes (less than approximately 0.140 millimeters). The flow rate is substantially independent of injector stroke at large strokes (typically greater than approximately 0.200 millimeters). In the transition region (approximately 0.140 millimeters to 0.200 millimeters), flow rate is determined by both the orifice and the injector stroke.
For small injector stroke, a slight variation in manufacturing tolerances during the assembly of the needle relative to the seat dramatically affects the flow rate. Other factors, such as wear in the needle or wear in the seat, can exacerbate this effect. This variation in the static flow rate is contrary to the purpose of a fuel injector, which is to provide accurate and precise fuel delivery to the combustion chamber. A known fuel injector is designed by first determining the desired output flow rate. Next, the orifice is sized to limit the flow rate to the desired value. The stroke is then set at a value that permits a flow rate that is greater than that permitted by the orifice.
Until recently, only liquid fuels have been used in internal combustion engines. However, environmental and natural resource concerns have introduced gaseous fuels to internal combustion engines. Gaseous fuels are generally less dense than liquid fuels. Therefore, a higher flow rate is needed to provide the same amount of fuel as compared to a liquid fuel system. Traditionally, the size of the orifice is increased, as compared to a liquid fuel injector, to provide this increased flow rate. A corresponding substantial increase in the injector stroke is required to prevent the frusto-conical area between the needle and the seat from influencing the flow rate. Substantially increased stroke introduces versatility, performance, durability and noise concerns to a fuel injector design.
However, it would be desirable to design a fuel injector in which the stroke alone determines the fuel flow rate. It would also be desirable to be able to design and manufacture fuel injectors with common components, but with different operational characteristics in which different strokes provide different fuel flow rates. The operational characteristics of a particular injector are determined by the manner in which the components are installed in the injector and interrelationships between components. Such designs facilitate mass-production assembly and reduce costs.
Briefly, the present invention provides a fuel injector comprising a housing having a hollow passage, a longitudinal axis extending through the hollow passage and a seat disposed at one end of the hollow passage. A needle is slidably mounted in the hollow passage and operable between a first position wherein the needle engages the seat and a second position wherein the needle is disposed away from the seat. A first flow area is formed by a minimum frusto-conical area between the seat and the needle when the needle is disposed away from the seat. The fuel injector also comprises an opening at the one end of the hollow passage proximate to the seat. The opening has a second flow area larger than the first flow area.
The present invention also provides a method for assembling a fuel injector having a housing and a valve disposed within the housing, the valve including a seat having an opening and a needle. The method comprises determining a desired fuel flow rate for the injector; providing the opening to permit a fuel flow rate greater than the desired fuel flow rate; and setting an injector stroke of the needle required to provide the desired flow rate.
The present invention also provides a method for generating different flow rates from each of a plurality of fuel injectors. The method comprises providing substantially uniformly dimensioned components for each of the plurality of fuel injectors; determining a desired respective gaseous flow rate for each of the plurality of fuel injectors; determining an injector stroke respective to each of the plurality of fuel injectors to provide the respective flow rate; and securing the components of the first of the plurality of fuel injectors in a first position to obtain a first injector stroke and securing the components of the second of the plurality of fuel injectors in a second position to obtain a second injector stroke.